1. Field of the Invention
The present invention relates to heaters that can heat enclosed areas both by radiation and by convection heat. These heaters have many uses. Particularly, such heaters have been used in the heating of glass sheets within a furnace, particularly where it is desired to support glass sheets either partly or entirely on a thin bed of hot gaseous material such as exists in the so-called gas hearth process. Examples of furnaces utilizing the gas hearth process for heating glass sheets may be found in U.S. Pat. No. 3,223,501 to Fredley et al, in U.S. Pat. No. 3,332,759 to McMaster et al, in U.S. Pat. No. 4,059,426 to Starr and in U.S. Pat. No. 4,059,427 to Starr et al. The latter two patents disclose a gas hearth furnace in which an air flow amplifier is used to recirculate gases that are used to heat and support glass sheets conveyed through the gas hearth furnace. The air flow amplifiers of these latter patents employ the Coanda effect to induce entrainment of a greatly amplified volume of recirculating gas flow with a relatively small flow of exterior air into the gas hearth apparatus. The compressed air that is introduced is heated en route to the air amplifier by applying electric voltage to a pipe supplying the air that is heated en route to the air flow amplifier.
The electrically heated tube radiantly supplies heat in U.S. Pat. No. 4,059,427 to radiate downwardly toward the upper surface of the glass sheet supported on a thin gaseous bed and the rate of air flow of the hot gas applied toward the lower surface of the glass sheet is augmented by the introduction of the heated air to entrain the recirculated air through the air flow amplifier of that patent. However, in the event that the introduced gas into the air flow amplifier is not sufficiently hot to raise the temperature of the recirculating gas within the gas hearth to compensate for the loss of temperature resulting from its heating the cool glass sheets that are conveyed through the gas hearth furnace, the application of additional electrical energy into the pipe portion that faces the upper surface of the glass may cause the upper surface of the glass to be heated to a greater amount than the lower surface of the glass. It would be beneficial to the glass heating art to develop an auxiliary heater that could be used to independently control both radiant heat and convective heat where the total heat is provided in a controlled manner to equalize the heat supply to the top and bottom surface of the glass sheets passing through the gas hearth furnace and the convective heat is independently controlled in such a manner as to help support the glass sheets in proper spaced relation above the upper surface of the gas hearth.